The vice presidential slot has tended to be the attack dog slot. I don't think Sen. Edwards did a good job of that in 2004 and Joe Lieberman was horrible at it. Al Gore was not bad. I would have to think that the vicious streak the Clinton campaign revels in would be much better directed at the GOP candidate. There are a whole host of negatives to having her on the ticket (a demoralized GOP base would love to have a reason to come vote out Clinton), but fight is not one of them.